Voltage measurements were performed by means of differently manufactured galvanic cells of the type Pt/Au, O-2, CO2, Me(2)CO(3)/Zr1-xYxO2-x/2 (YSZ)/O-2, (CO2), Pt with Me = Li, Na, K. The results indicate that the cell voltage is determined by the Me activity and by the activity of the oxide Me(2)O dissolved in the carbonate. Since both of these quantities equally depend on the CO2 partial pressure of the surrounding atmosphere and since the carbonate readily allows the oxide chemical potential to become uniform over the layer thickness, the cells do not respond to changes in gas composition. The considerations as to the electrode reactions prove to be reasonable by additionally applying an Me-silicate interlayer between Me(2)CO(3) and yttria-stabilized zirconia (YSZ), which enables adjustment of the Me(2)O activity independently of CO2 pressure, thus providing a satisfactory CO2 response. The present study gives evidence that (i) the functioning of the Au, O-2, CO2, Me(2)CO(3)/YSZ electrode is not bound to the presence of a so-called ionic bridge as discussed in the literature and (ii) the electrode reactions together with the transport properties of the carbonates make the Au, O-2, CO2, Me(2)CO(3)/YSZ electrode unsuitable for use in a CO2 sensor.